Glass substrates may be used in a variety of applications, ranging from windows to high-performance display devices. The quality requirements for glass substrates have become more stringent as the demand for improved resolution, clarity, and performance increases. Glass quality may, however, be negatively impacted by various processing steps, from forming the glass melt to final packaging of the glass product.
One processing step that may result in reduced glass quality is the melting process, wherein glass precursor batch materials are mixed and heated in a melter. During this process, the materials melt and react, giving off reaction gases, which produce bubbles in the molten glass. Additionally, the melting process may produce an inhomogeneous glass melt having regions with differing chemical compositions. The first melt to form is often highly reactive with the refractory materials, which may lead to excessive wear of the apparatus and/or defects in the glass melt. Denser portions of the melt may also sink to the bottom of the melter, leading to a “sludge” layer which has different optical properties than the rest of the melt and is difficult to completely mix back into the overall melt, which results in inhomogeneous portions of the melt, referred to in the art and herein as “chord.” Finally, due to the typically large processing volume, it is possible that various glass batch materials may not melt or may only partially melt. The un-melted or partially melted materials are carried through the melting process and may later become defects in the glass product.
Current melting processes for producing high quality optical glass utilize high temperatures and stirring to remove bubbles from the glass melt. However, such processes may be cost prohibitive, as they require expensive metals and specially designed high temperature refractory materials for the processing equipment. Further, these costly melting systems require a long processing time and high energy expenditure as the reaction gases have a long distance to travel to escape the glass melt and the sludge layer must be mixed from the bottom of the melter tank into the rest of the glass melt in the tank, requiring a mixing motion over a long distance through a highly viscous fluid.
Alternative methods for preventing glass bubbles and inhomogeneous portions in the glass melt include processing the melt in smaller batches. In this manner, the gas bubbles have a shorter distance to travel to escape the melt and the sludge layer can be more easily incorporated into the rest of the melt. However, as with many small scale processes, these methods have various drawbacks such as increased processing time and expense.
Significant issues have also been observed when trying to melt various “unconventional” glass precursor materials. For example, glass batch materials comprising a mixture of barium oxide and alumina or a mixture of calcium oxide and alumina may have extremely high melting temperatures, e.g., above 2,100° C., which complicates the melting process and makes it difficult to achieve a homogeneous glass melt. Other glass batch materials, such as mixtures of silica, sodium oxide and calcium oxide can be melted, but have a tendency to crystallize upon cooling, and require special quenching methods to reduce the possibility for processing defects. Ultra-low expansion (ULE®) glasses comprising silica and low levels of titanium dioxide may also present difficulties when melted using traditional processes.
Accordingly, it would be advantageous to provide glass melting processes which are faster and/or more economical, while also minimizing issues relating to glass quality, such as defects caused by bubbles, chord, and/or striae in the melt, and which may be suitable for melting unconventional glass materials. The resulting glass substrates can possess high optical qualities and can be used in various commercial products requiring glass with high resolution, clarity, and/or performance.